Can I apply for a Non-O marriage visa while still working and earning a monthly income?

You need to go to your local immigration office to get the latest requirements for Non-O in-country or extension.

Are you already on 1-year extension based on employment?

If so, you can apply for 1-year extension based on marriage to a Thai (changing the reason) when your next application by meeting the requirement for an extension without leaving Thailand.

Unless I'm mistaken you need to get certification of employment (or long-term employment contract) from your employer to meet the requirements for Non-O in-country or 1-year extension along with other documents from them to use monthly income inside Thailand. It'd be hard to get if you are leaving the company.

But Once you are granted 1-year extension, you don't have to show the money until next 1-year extension application.

If you cannot get the documents from your employer, you need to have 400K in your Thai bank account to apply for changing the reason for a 1-year extension based on marriage.

IF you are still on the first 90 days from your Non-B entry/start, you need to leave once to be able to apply for a new Non-O based on marriage to a Thai. Basically, financial requirements are same as above.